Quasi-degeneracy effects

Quasi-degeneracy Effects.—The convergence properties of the non-degenerate formulation of the many-body perturbation theory deteriorate when quasidegeneracy is present in the reference spectrum. In view of its simplicity, however, there is considerable interest in exploring the range of applicability of the nondegenerate formalism. 

In order to eliminate the arbitrary use of the model and shifted perturbation series, depending on the degree of degeneracy in the corresponding reference spectra, the following zero-order hamiltonian has been considered 148 

Obviously, when severe near degeneracy is present in the reference spectrum, a full quasi-degenerate formulation of the perturbation series should be employed. However, there is considerable value in applying the much simpler non-degenerate formalism to as wide a range of problems as possible. 

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B. Datta, D. Mukherjee, Treatment of quasi-degeneracy in single-reference coupled-cluster theory. Separation of dynamical and nondynamical correlation effects, Chem. Phys. Lett. 235 (1995) 31. 

The dynamic electron correlation means the rest of the correlation effect, beyond the static one. It corresponds also to occupying orbital energies, but not those related to the degeneracy or quasi-degeneracy of the ground state. As we see, the distinction between the static and the dynamic correlation is a bit arbitrary. 

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